1. Field of Invention
The present invention relates to methods for selecting lubricants used in heat pumps. More particularly, the invention relates to methods used to select lubricants and hydrofluoroolefins refrigerant combinations for use in compressor-type heat pumps.
2. Description of Prior Art
The use of chlorine-containing refrigerants, such as chlorofluorocarbons (CFC's) and hydrochlorofluorocarbons (HCFC's), in a compression-type refrigeration device (e.g., heat pumps, air conditioners, refrigerators, and the like) is disfavored because these refrigerants can damage the Earth's ozone if they leak or are otherwise discharged from the device. Accordingly, it is desirable to retrofit chlorine-containing refrigeration systems by replacing chlorine-containing refrigerants with non-chlorine-containing refrigerant compounds that will not deplete the ozone layer, such as hydrofluoroalkanes (HFCs) or hydrofluoroolefins (HFOs). Of these, HFOs are more desirable because they are typically characterized as having a much lower Global Warming Potential (GWP).
Preferably, these replacement refrigerants are compatible (e.g., miscible) with conventional compression-type refrigeration device lubricants. Such compatibility allows lubricant to flow more easily with the refrigerant throughout the system thereby increasing the system's efficiency and expected life span. The lack of compatibility can result in separation of the refrigerant and lubricant into different phases. If phase separation occurs between the refrigerant and lubricant while the refrigerator is running, it affects the life and efficiency of the apparatus seriously. For example, if phase separation of the refrigerant and the lubricating oil occurs in the compressor, the moving parts would be inadequately lubricated, resulting in seizure or other troubles and thereby the life of apparatus is shortened considerably. If phase separation occurs in the evaporator, a lubricating oil having high viscosity exists and thereby the efficiency of heat exchange is decreased. Unfortunately, many non-chlorine-containing refrigeration fluids, including HFCs and HFOs, are relatively insoluble and/or immiscible in conventional lubricants, including, for example, mineral oils, alkylbenzenes or polyalpha-olefins. In order for a refrigeration fluid-lubricant combination to work efficiently within a compression refrigeration, air-conditioning or heat pump system, the lubricant is preferably compatible with a refrigerant over a wide range of operating temperatures.
Generally, a compression-type refrigeration device is composed of a compressor, a condenser, expansion valve, and an evaporator, having a mechanism wherein the mixture of a refrigerant and a lubricating oil is circulating in the closed system. In said compression-type refrigeration device, though it depends on the kind of apparatus, generally the temperature in the compressor rises to about 50° C. or higher, while in the cooler, the temperature comes to be about −40° C. Accordingly, the refrigerant and the lubricating oil must circulate in this system without phase separation usually in the range of about −40° to +50° C. (See, e.g., U.S. Pat. No. 5,536,881, which is incorporated herein by reference.)
Accordingly, the selection of a lubricant for a compression-type refrigeration device should include an analysis of the lubricant's miscibility with the desired refrigerant. Achieving miscibility reduces the need for an oil separator, which can be installed immediately after the compressor to capture immiscible oil and return it to the compressor to maintain a desired level of lubrication. Maintaining good lubrication reduces problems associated with low oil return, including fouling of the heat exchanger surfaces and burn-out of the compressor. However, as noted above, many desirable HFC and HFO refrigerant are immiscible in conventional lubricants at room temperature. Although various mixtures of HFO and lubricants are known (see, e.g., US Patent Application Publication No. 2004/00898, which is incorporated herein by reference in its entirety), there remains a need for a method of selecting a lubricant that is compatible with HFC, and particularly HFO, refrigerants.